解决Raspberry PI 4 ssh远程连接时connect refused

直接参照官方文档，写的很清楚：
https://www.raspberrypi.org/documentation/remote-access/ssh/

2. Enable SSH
   As of the November 2016 release, Raspbian has the SSH server disabled by default. It can be enabled manually from the desktop:

sudo raspi-config
then
Select Interfacing Options
Navigate to and select SSH
Choose Yes
Select Ok
Choose Finish

或者直接使用以下命令：

sudo systemctl enable ssh
sudo systemctl start ssh